During inspection or servicing of pneumatic or hydraulic systems employing solenoid-operated pilot valves, it is often desirable to be able to determine if a particular pilot valve solenoid is actually electrically energized as and when it is supposed to be. Sensing the electrical condition of such solenoids has heretofore been accomplished by either inserting a suitable detector in the solenoid circuit at the time of the inspection, such as by disconnecting a wire at the solenoid and temporarily connecting in a meter, or by having a light permanently wired in the solenoid circuit. The first approach was time consuming and the second substantially raised the initial cost of the solenoid.
The following prior art U.S. patents show sensing devices either hard wired into a switch or solenoid circuit or inductively coupled with a carrying wire:
U.S. Pat. No. 2,591,336 PA1 U.S. Pat. No. 2,611,843 PA1 U.S. Pat. No. 3,316,447 PA1 U.S. Pat. No. 3,488,611 PA1 U.S. Pat. No. 3,909,670 PA1 U.S. Pat. No. 5,015,944